1. Field of the Invention
The present invention relates to a non-contact memory card system that is configured so as to perform sending and receiving of data by means of electromagnetic coupling (known also as electromagnetic induction), and more specifically to a storage medium system that uses a contactless memory card which is configured so as to perform sending and receiving of data by means of electromagnetic coupling (known also as electromagnetic induction) via an LC resonant circuit.
2. Description of Related Art
In recent years, work on achieving practically usable contactless memory cards for as a small, highly reliable storage medium has progressed, resulting in an expanded range of applications thereof, as well as a significant increase in their storage capacity.
Along with the achievement of large storage capacity, there is a naturally occurring need to transfer data at high speed.
In general, however, it is extremely difficult to increase the speed of data transmission when performing transmission using an electromagnetic coupling via an LC resonant circuit.
Methods of increasing the transmission speed which can be envisioned include such methods as (1) simply decreasing the amount of time required to send each bit, (2) providing a plurality of LC resonant circuits so as to send data in parallel, and (3) employing multiphase modulation so as to send a plurality of data bits simultaneously.
The last method (3) requires a plurality of modulator and demodulator circuits for multiphase PSK or multiphase FSK, making its employment particularly difficult in the field of contactless memory card systems, in which there is a strong demand for compact, lowcost readers and writers.
A known storage medium system that makes use of such a contactless memory card, such as that shown in FIG. 8, has a reader/writer-side electromagnetic coupling interface section 1 and a memory card side electromagnetic coupling interface section 10.
FIG. 8 is the circuit diagram of the communication circuit of a contactless memory card system of the past, and FIG. 9 is a drawing that shows the main waveforms in therein.
In a storage medium system which uses the contactless memory card of the past as shown in FIG. 8, a printed coil type antenna 2 of the reader/writer-side electromagnetic coupling interface 1 and a printed coil type antenna 12 of the LC resonant circuit 11 of the memory card side electromagnetic coupling interface 10 are in opposition to one another when access is performed, bringing these elements into electromagnetic coupling condition so that data can be passed therebetween.
That is, the antennas 2 and 12 on the reader/writer side and card side, respectively, are both used for both sending and receiving.
If the start-stop synchronized serially transmitted data BD having a bit time width (time length of 1 bit) of tb shown in FIG. 9 is sent by the card side to the reader/writer side, corresponding to the time when the transmitted data BD is 0, at the transmitted signal forming circuit 15, a transmitted signal TX is formed by a single-shot signal, this signal driving the transmitting driving transistor 13.
That is, each time the above-noted transmitted data DB is 0, the resonant circuit 11 is single-shot driven, resulting in electromagnetic induction in the antenna 2 of the reader/writer-side electromagnetic coupling interface section 1.
When this occurs, although the drive is single-shot, because of the resonating phenomenon in the resonant circuit 11, the current waveform in the antenna 12 is a freely damped waveform, the induced waveform in the antenna 2 of the reader/writer-side interface section 1 also exhibiting a similar freely damped waveform shape.
The above-noted induced waveform occurring in the antenna 2 is input to the receiving demodulation circuit 3 at which it is demodulated, the DC component being first cut from and bias being applied to this signal by means of capacitor C1 and resistors R1 and R2, thereby becoming the input signal WP which is then input to a window comparator 6 (this being the signal with respect to which receiving demodulation is performed).
Therefore, while the waveform of the input signal WP is approximately similar to the waveforms (not shown in the drawing) that appear in the antennas 12 and 2, if this input signal WP exceeds the limits of a window W that is established by the resistors R3, R4, and R5, the receiving demodulated signal RD that is the output of the window comparator 6 is made 0.
If this receiving demodulated signal RD is at the 0 level for even one instant during a prescribed sampling period tc which has as its relative phase reference point the detected point TR0 of the start bit thereof, at the received data processing circuit within the reader/writer, the received data during this period is treated as 0.
If the start-stop synchronized serially transmitted data TD is transmitted from the reader/writer side to the card side, when the transmitted data TD bit data is 0, a transmission carrier TC of a prescribed frequency is input to the transmission driving circuit 5 via the transmission modulator circuit 4.
That is, the transmission modulator circuit 4 performs inverted ASK modulation (in which the carrier signal is output when the transmitted data is 0) of the transmission carrier signal TC in accordance with the transmitted data signal TD, and during the period in which the transmitted data TD is 0, the series resonant circuit formed by capacitor C2 and the antenna 2 of the transmitting driving circuit 5 is driven, the resonant waveform RS appearing as a result of induction therefrom in the card-side resonant circuit 11.
Because this resonant waveform RS is input to the detector circuit 14 at which it is detected, when the antenna 2 is driven by the transmission carrier signal TC, the received data RX output from the detector circuit 14 is basically also 0.
Each prescribed sampling timing TSP of the received data RX, which is established using the start bit detection point TS0 thereof as a phase reference, serial received data is captured, one bit at a time.
As described above, in a contactless memory card system of the past, as shown in FIG. 8 and FIG. 9, receiving at the card side is performed via an LC resonant circuit, this being effective in simplifying the receiving circuit at the card side so as to enable the reception of a maximum amplitude signal with respect to the signal transmitted from the reader/writer.
Additionally, because noise at frequencies other than the resonant frequency of the LC resonant circuit is not easily accepted, this has the advantage of suppressing communication errors caused by noise from outside the system.
When transmitting from the card side, using a transmitted signal that is formed by a single-shot signal so as to drive the above-noted resonant circuit for transmitting is effective in minimizing the amount of electrical power required when transmitting from the card.
However, in spite of the above advantages, because of the characteristic free damping in the above-noted resonant circuit, the phenomenon of the lingering of the transmitted signal presents problems in terms of both communication reliability and improving the speed of transmission.
Specifically, in the case in which transmission drive is done by a single-shot transmitted signal, because of the free damping in the resonant circuit, there is not sufficient reduction in the amplitude thereof during the bit time width tb of the serial transmitted data BD, this resulting in the risk of causing a receiving error at the reader/writer side.
For example, as shown in FIG. 9, if we assume the case in which a the time TS the bit data is 0 and the transmitted signal is output, and in which at the time TE the bit data becomes 1 and the transmitted signal is not output, if the amplitude when the sampling period tc is reached with respect to the time TE, because of the lingering freely damped signal in the resonant circuit (this being referred to as reverberation hereinafter), still exceeds the window, a communication error will occur at this point.
In a communication circuit of such a contactless memory card of the past as described above, because of reverberation in the LC resonant circuit, there is a loss of communication reliability, and in order to improve the communication reliability it is necessary to reduce the communication speed so that the bit period is long, so that the communication period for a bit is started only after the reverberation of the previous bit has sufficiently settled.
If the drive power at the transmitting side is merely increased to make the communication waveform amplitude large, the reverberation amplitude also becomes large, so that this is clearly not a solution to the problem of reliability margin or improvement of transmission speed.
As described above, in the prior art there was a limit to increasing the communication speed without sacrificing communication reliability, and if it were possible to achieve high-speed communication, this would lead to the microprocessor that controls the reader/writer for card access not being able to keep up to the operating speed, or losing received data or experiencing overrun errors while it was processing emergency interrupts.
That is, in a contactless memory card system, because the number of communication transmission paths between a card and a reader/writer is limited to the minimum because of limitations such as those on space, it is extremely inappropriate to provide, as is done in communications relying on direct connections, a dedicated signal line for handshaking, this being used to make the other device wait for transmission.
As a result, in a contactless memory card system of the past, there was a need to enable the host system microcomputer to successively read in data via a reader/writer without overrun errors when the card side first starts sending data read from memory.
Therefore, although card reader/writers use a variety of microcomputer types, depending upon the application system, some of these microcomputer types have slow operating speed, and when use with such slow microcomputers is considered, it ultimately becomes impossible to increase the speed of transmission between a contactless memory card and the reader/writer.
Depending upon the application, during receipt of a transmission from the card, there is a possibility of receiving an interrupt having a higher priority, and in this case if the next data is received before the microcomputer captures data received by the reader/writer, an overrun error occurs.
An object of the present invention is to provide a contactless memory card system having a contactless memory card and a card reader/writer that perform mutual passing of data therebetween via an electromagnetic coupling interface section that includes a coil that serves for both transmitting and receiving, this system overcoming the above-noted problems associated with the prior art, so that it is possible to increase the speed without causing overrun errors even when connected to a microcomputer having a slow operating speed, if the microcomputer side performs processing of an interrupt, leading to the possibility that the capture of data from the reader/writer will not be done on time.
A further object of the present invention is to provide a contactless memory card configured so as to perform data transmission by an electromagnetic coupling via an LC resonant circuit, this solving the above-noted problem associated with the prior art attributed to reverberation in the above-noted resonant circuit, enabling an increase in the communication speed without sacrificing communication reliability.